Continuous production of a coated web often requires the web to be coated with a substance made liquid or semiliquid by including a volatile solvent. The freshly coated web is then passed through a heating zone where the solvent is driven off, after which the hot web is wrapped around one or more rotating cooling rolls and brought down to a temperature permitting further processing.
For example, when using the offset printing process for printing a continuously traveling paper web, the freshly printed web is continuously passed through a drying oven normally operated under sub-atmospheric pressure, or under-pressure, and where the ink's solvent oil is vaporized and removed to an extent normally permitting the web to be further processed without smudging of the ink. However, the hot printed web leaves the oven as a continuously traveling web with its printing containing a residual amount of the oil which continues to vaporize. If the oven temperature is raised enough to prevent this, the paper itself is undesirably dried of its natural moisture content. The web and its ink are hot and require cooling for further processing.
For compactness of the required cooling equipment, it is desirable to wrap the traveling web around a first cooling roll so that the web is traveling backwards and next around a second cooling roll in front of the first which provides further cooling and again reverses the travel of the web so it is again traveling forwardly. The web wraps only partially around each roll such as approximately somewhat over 180.degree., leaving the balance of the cold roll surface exposed.
The traveling web tends to carry the vapor of the still vaporizing residual solvent oil along on its surface in the form of a boundary layer conveyed by the web motion, and when this layer of vaporizing solvent oil contacts the exposed portions of the cooling rolls, it can condense to its liquid phase on the roll surfaces and cause a solvent or softening action on the otherwise adequately dried ink, resulting in smudging of the ink by one or the other of the rolls and a defective printed product. Heretofore, the only remedy was to raise the temperature of the drying oven with the undesirable consequence of excessively drying the paper itself so that it no longer retained its natural moisture content.
It is possible to use a normal drying oven temperature if the web leaving the oven is cooled in the form of a mechanically unsupported span, but this involves the use of equipment having an undesirable length. It is more desirable to use the two cooling rolls because this permits the use of shorter equipment.
In the present instance, the object is to provide an apparatus enjoying the compactness permitted by the cooling rolls while avoiding the ink smudging possibility described above.